A yarn draw-off device on a rotor spinning machine is constituted by a pressure roller and draw-off shaft. The invention relates to adjusting the contact pressure of a pressure roller against the draw-off shaft of a yarn draw-off device. The contact pressure is necessary in order to draw the yarn between rotating pressure roller and rotating draw-off shaft out of the rotor and to convey it in the direction of the winding device.
Rotor spinning machine are equipped with a plurality of individual spinning stations which produce a yarn from fiber sliver. The yarn must be withdrawn continuously from each individual rotor by means of draw-off devices and delivered to a winding device where it is wound into a yarn bobbin. The draw-off device of a rotor spinning machine consists of a draw-off shaft or its segments which are axially rotatable on each machine side. Both draw-off shafts are driven and are imparted their rotational movement. The corresponding drive is located in the drive frame of the rotor spinning machine.
The draw-off device is completed by the pressure roller, whereby one pressure roller assigned to run the yarn at a spinning station on the draw-off shaft is applied to the draw-off shaft. The individual pressure roller is supported by a pressure roller lever which is attached to the machine frame. The pressure roller is rotatably mounted on the pressure roller lever. The pressure roller rotates as it lies on the rotatable draw-off roller. For piecing or in a piecing operation, i.e. when a yarn breakage is being repaired, the pressure roller can be swivelled together with its pressure roller lever away from the draw-off shaft into an open position and can be swivelled back from there into its operating position. The yarn can thus be placed in the yarn draw-off device and can be clampingly held.
The draw-off roller together with the pressure roller draws the produced yarn out of the rotor groove through the draw-off pipe of the spinning box. In order to ensure the regularity of a yarn number, the yarn must have little slip, i.e. it must be drawn off at a constant speed from the rotor groove. In order to ensure this state, the pressure roller of each spinning station is pressed against the draw-off shaft with a constant force. The contact pressure of the pressure roller is produced in the state of the art by means of a tension spring. The tension spring is held on one side in a fixed angle bracket of the machine frame and pulls on the other side the pressure roller with a constant force via the pressure roller lever against the draw-off shaft.
Different values for a constant contact pressure would be required as a function of the yarn number in order to withdraw the yarn with little slip. This requirement is simplified in that the contact pressure is adjusted one time for the highest yarn number that can be produced and for the requirements in the piecing process. The tension spring used produces this constant contact pressure for the highest yarn number produced. This contact pressure is not changed.
This has however the disadvantage that the energy utilization for yarn withdrawal for all the other yarns is always over-sized with respect to ensuring low-slip yarn withdrawal at constant speed. Since this over-sizing of the energy utilization during yarn withdrawal applies during the entire production time, the resulting loss in energy reaches a magnitude that is economically felt.